Flat glass is utilized as windows for buildings and vehicles. The glasses so utilized have a relatively flat transmission curve across the near infra-red, visible, and ultra-violet portions of the electro-magnetic spectrum. For many applications, it would be desirable to reflect some portions of the spectrum, and transmit other portions, either with or without attenuation. Various pigments, dyes, and metallic color coatings have been used to these ends. For example, high and low refractive index coatings have been used as anti-reflection coatings, i.e., to increase transmission, while multi-layer coatings, dyes, and chemical modification of the subjacent glass have been used to impart color.
High and low refractive index materials have heretofore been used as neutral color anti-reflective coatings on transparent substrates. First surface, multi-layer anti-reflective coatings are described, for example, in U.S. Pat. Nos. 3,410,625 to Edwards, 3,781,089 to Cicotta, 3,958,042 to Katsube and 3,176,575 to Socha. Edwards describes a repeating structure of dielectric layer pairs, one member of the layer pair being a high refractive index material, and the other member of the layer pair being a low refractive index material. The layer thickness is EQU L/4n
where L is the wavelength sought to be reflected, and n is the refractive index of the light of wavelength L in the medium.
Cricotta, et al describe an anti-reflecting neutral density filter made up of a repeating structure of layer pairs on the first surface of a glass body. One member of the layer pair is a high refractive index metal, and the other member is a low refractive index dielectric. The alternating layer pair structure provides a non-colored, neutral density filter of desired optical density and reflectivity.
Katsube, et al describe an anti-reflecting coating on the first surface of a glass body. The coating has alternating high and low refractive index layers. Each layer has a thickness on the order of 0.0125 to 0.050 wavelengths. The anti-reflective coating is color free.
Socha describes a low reflectance multilayer for the first surface of glass. The multilayer includes a SiO.sub.2 layer approximately 1/32 to 1/16 wavelength thick on the glass, and high refractive index layer approximately 1/2 wavelength thick on the SiO.sub.2, and an intermediate refractive index coating approximately 1/2 wavelength thick atop the high refractive index layer. The resulting colorless coating is an anti-reflective coating.
U.S. Pat. Nos. 4,188,452 to Groth and 3,411,934 to Englehart utilize first surface multiple oxide coatings for architectural effect. Groth uses layers of SiO.sub.2 and TiO.sub.2 to reflect ultraviolet light. Englehart uses multiple layers of cobalt oxide and tin oxide on the first surface of glass to provide a color reflecting coating.
U.S. Pat. No. 4,170,460 to Donley describes a method of making colored glass articles by the migration of optically active metal ions into the first surface of a chemically tempered glass substrate, thereby forming a colored, high refractive index zone within the glass. This metallized zone has a higher refractive index then the subjacent glass. A metal layer is then deposited above the treated glass and completely oxidized to form a coating of higher refractive index. The resulting oxide coating interacts with the chemically tempered, metal-ion containing glass to form an interference film.
The use of first surface reflective coatings of high refractive index oxides is disclosed in published British patent application No. 2,063,920 of J. P. Coad, et al. Coad, et al describes production of reflective surfaces by ion beam sputtering of enumerated transition metals onto a substrate and subsequent anodizing of the sputtered metal to form a high refractive index oxide thin film, capable of interference. However, Coad, et al do not disclose partially transparent coatings. Their coatings are opaque. Nor does the Coad patent disclose thermal oxidation or plasma oxidation of the sputtered metal film.